This invention relates generally to a prosthetic joint and, more particularly, concerns a hinge type prosthetic joint for replacement of a natural joint in the human body especially when relative movement of the components of the joint is generally in one plane.
Replacement of defective, deteriorated or otherwise nonfunctioning natural joints by mechanical prosthetic devices has become more prevalent due to the recent advances in the specific designs of the prostheses and the surgical techniques to implant them in the patient. Individuals who have joints with limited or no movement, accompanied by pain and discomfort, such as with severe rheumatoid arthritis may undergo a surgical treatment to replace the affected joint, including treatments such as resection arthroplasty. In resection arthroplasty, the articulative portions of the natural joint are removed, and the bones are prepared to receive the prosthesis as a substitute for that particular joint. In addition to relief of pain and discomfort, it is expected that the prosthesis will allow the individual to regain use of the affected joint in a way which resembles, as close as possible, the use and function of the original natural joint. Factors such as relative movement, stability, strength compatibility in the human body, durability and the like must be taken into consideration in the design of the prosthesis. The goal of designing the prosthesis is to achieve a mechanical joint which, although a substitute, functions substantially equivalently to the natural joint which it replaces.
In a number of joints in the human body, the bone components which form the same have movement with respect to each other generally in one plane, i.e., one bone swings in a rotative movement with respect to the other in a hinging effect. This type of movement is found, for example, in the finger joints, such as the proximal interphalangeal (PIP) joint. Although there may be some rocking movement or action in the natural PIP joint, the rotative movement of the bones forming the joint is generally in one plane. Many prostheses which are designed to replace, for example, the PIP joint or similar type joint, include an axle which is used to pin together the components of the joint and thereby allow hinge-type rotation of the components. In these hinge-type prostheses, not only is the axle or pin relied upon to provide a pivot point, but also to act as a bearing surface to carry the loading forces of the joint in use. Such a prosthetic joint wherein an axle is employed for hinging purposes and also as a load bearing member is typified in U.S. Pat. No. 3,879,766. Inasmuch as the axle is generally a slender member, it is undesirable to place reliance on the axle to carry unexpectedly large loading forces or to possess durability during the period of time the prosthesis is implanted. In this regard, hinge-type prostheses which include an axle or pin to hold the components together, and which relay upon the axle as a bearing surface to dissipate the loading forces of the joint, may be somewhat weak from the standpoint of joint strength. It can be seen that the search for mechanical replacements for particular natural joints remain viable insofar as improvements in the particular designs are still sought.